MOOG / Gibson Lab L7 Pop

[plexi50]

I have this annoying pop whenever i turn the amp on. It pops pretty good. Enough to make me worry about the speakers taking a surge spike in voltage but i have not seen any abnormal voltage on the speaker when i turn the amp on when it does this pop thing at power up. I have had quite a few Lab L7-L5 L9 L11 Series amps over the years. I had to buy another one. So here are my search results so far.

I have narrowed down the cause to the preamp board.  I have changed all the electrolytics on the power supply and reverb board except for the big 3000uf / 75 volt 100 volt surge can capacitors. I checked them with my old analog tester for saturation and they bleed right down. (don't mean there good though) No amp hum. I have changed the Compressor diodes CR101 & CR102 and power supply /reverb board. The 4558 chips on the input of the preamp input are working but seem to be where this pop is coming from. Not 100% certain though.

A pop on powering off is normal on some amps but this power on pop is a pretty good POP!
All power transistors (tubes) are good. Could the JRC4558 chips produce a pop if they are bad? They either work or they don't to me.

Anything is possible. But i thought i would have found the problem by now. I would like to find the 3000uf 75 volt 45mm diameter can capacitors but that has been virtually an obsolete part issue and any subs i find  are 35 mm in diameter. The screw terminal spacing as well has to be correct. I have not measured them yet. Great sounding amp otherwise!

[PRR]

The turn-on POP will not damage a guitar speaker. FULL-out wailing is just as strong and lasts longer.

Some of us have not memorized all schematics for all amps. Give link.

[plexi50]

The turn-on POP will not damage a guitar speaker. FULL-out wailing is just as strong and lasts longer.

Some of us have not memorized all schematics for all amps. Give link.

Schematic & service manuals here. Too big to post here.

http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg

http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html

[shooter]

If you have a scope connect it to the tip of pre out or PA in jack.  set it for VDC 10v/div, no signal, time base set for like viewing a KHZ freq, then power up. What I'm guessing is you'll see the dc jump, WITH a large transient spike, maybe 10X normal jump from 0 to 60? VDC.
Without a scope you could line-out, line in to 1/2 split the circuit

[PRR]

Put a shorted plug in the "Power Amp Input" jack. That separates the preamp from the power amp. Does it still pop?

[plexi50]

Put a shorted plug in the "Power Amp Input" jack. That separates the preamp from the power amp. Does it still pop?

Shorted jack plugged into power amp. Pop is gone on power up now. Only very slight pop on shut off.

[PRR]

So it is preamp.

Unfortunately there is a *LOT* of preampery there. I would look for places where signal can be broken, turned-down, or temporarily shorted, to narrow-down the offending section.

MASter Volume is an obvious spot. If still pops, suspect the compressor to the right of that. It has an OFF switch, but a fault might still pop. Or it may just be working "normally"?

[plexi50]

I will start with disconnecting  the compressor circuit and see what happens and go from there.

[plexi50]

I have replaced (2) 1N4148 diodes in the compressor area of the preamp board.
I have ordered the (6) electrolytics for the preamp & power transistor board.
Removing and testing old preamp caps later today.

 

[plexi50]

Electrolytic Compressor cap and (1) other preamp cap saturated and do not bleed all the way down on analog meter.
One starts to rise again after nearing 0.
I wont have the new caps until at least Thursday of this coming week.

[shooter]

One starts to rise again after nearing 0.

I believe a discharged cap will RE-charge if hooked to a meter.

[plexi50]

One starts to rise again after nearing 0.

I believe a discharged cap will RE-charge if hooked to a meter.

Cap is checked on both pos & neg polarity and then reversed giving the same climbing above zero results after settling on close to zero but never quiet settling all the way down to zero.
In my experience if a electrolytic cap bleeds down and stays there it is good. If a cap starts to recharge after it has bleed down and the meter rises again and stays a distance floating above the zero ohm setting it is saturated and or a short is present. That's just my thinking on it.
That is a good question though to get the experts answer. My old Micronta  analog meter comes in handy for some things a DVM cant do.

[PRR]

Electrolytic caps, charged then shorted, always "come back" a little.

Go through your stockpile and try it.

[sluckey]

In my experience if a electrolytic cap bleeds down and stays there it is good. If a cap starts to recharge after it has bleed down and the meter rises again and stays a distance floating above the zero ohm setting it is saturated and or a short is present. That's just my thinking on it.

Your observation of the needle movement is correct, but you are thinking backwards about what is happening.

When you connect a good discharged cap to the ohm meter, the cap charges exponentially up to the battery voltage through the internal resistance of the meter. The charging current is initially big then tapers off as the cap charges. This will quickly swing the needle toward zero ohms (a big electrolytic will probably slam the needle to zero pretty hard) then as the charging current decreases the needle will drop back toward some higher resistance and settle at some steady resistance value. This indicates the cap has charged to the maximum voltage available and is holding that charge. This is the action of a good cap that is not leaky. That's what you expect to see with a good cap. A leaky cap will act the same ***EXCEPT*** rather than settle at some steady needle indication, the needle will begin to drift back towards zero ohms as the cap tries to keep charging. This is a leaky cap and should be replaced. Another indication of a shorted cap is the needle quickly swings toward zero or some low ohms but never begins to drop back towards a higher resistance. And one more bad indication is the needle never moves. This indicates an open cap and for electrolytics, this usually means it's dried out and no longer good.

None of the actual resistance readings are actually useful. It's the relative needle action that's important. The readings will be different for different caps, different range scales, and different meters. But the relative action is always the same. A good cap will quickly kick the needle toward zero ohms then drop back to some higher resistance and settle down at some steady reading. The cap has charged to the maximum voltage available and is now just sitting there.

[plexi50]

Great reading. I understand the testing process what you are saying better. I'm going to test them again now for fun and pay close attention to what happens again.

[shooter]

Did  turning the volume pots on the preamp to zero have any effect?  It looks like you can eliminate the reverb by un-plugging P22 feeding the verb board.

[plexi50]

Did  turning the volume pots on the preamp to zero have any effect?  It looks like you can eliminate the reverb by un-plugging P22 feeding the verb board.

No i eliminating the reverb and CCW rotated all the pots including the Master volume but the POP! was still present.
If it were a little pop i wouldn't be all this concerned about it. But you can see those 410 speakers jump when the amp is initially turned on.
I could ignore that but then again i cant. I should have all the caps this week and go from there.

[plexi50]

Capacitors arrived this morning. New capacitors installed. Power on pop is down to a much lower level that i feel is no longer a freak out issue. Now if i could only find those 3000uf / 75 volt 45mm power supply can capacitors. This amp sounds great!

[shooter]

Glad you got it.  you might be able *hollow out* the old cans and stuff in some modern mini-caps, silicon and walla!

[plexi50]

Glad you got it.  you might be able *hollow out* the old cans and stuff in some modern mini-caps, silicon and walla!

Brilliant idea Shooter! I did that with a 70's Princeton reverb a while back. I love the tone these old Lab Series amps have.
Thanks for your help. 

[Willabe]

So did BB King and Ronnie Montrose. (album with Town Without Pity, instrumental, he used the amps compression circuit for the long note sustain.) 

[plexi50]

I really like a clean full sounding amp. Just add a few good pedals and your set.